Turnover of intracellular components (soluble proteins and organelles) in lysosomes, a process known as autophagy, is essential for normal cell function. Three different types of autophagy have been described in hepatocytes: macroautophagy (MA), microautophagy (mA) and chaperone-mediated autophagy (CMA). Altered autophagy has been linked to common human pathologies, including those affecting the liver (protein conformational disorders, metabolic disorders, cancer and diabetes), and autophagic activity decreases with age in almost all tissues and organs. We have recently found that MA and CMA can compensate for each other, supporting the existence of cross-talking between these two autophagic pathways, but they are not completely redundant. Altered autophagy also results in changes in endocytosis, the pathway for delivery of plasma membrane and extracellular components to lysosomes. The overall goal of this proposal is to elucidate the basis for the cross-talk among autophagic pathways and of their interactions with the endocytic system. We will determine the participation of different subpopulations of lysosomes in each type of autophagy and in endocytosis and characterize vesicular fusion interactions among compartments in different pathways. We will use both in vitro fusion assays with high purity organelles isolated from rodent livers or hepatoma cells in culture, as well as real-time microscopy in cultured cells, to track and characterize vesicular fusion events and delivery of cargo to lysosomes. Using different cellular and animal models with altered autophagy or endocytosis, we will analyze possible compensatory changes in the other pathways and the consequences of interfering with these compensatory mechanisms in cell survival and in normal function. Understanding the interactions between the autophagic pathways and endocytosis would be essential for future efforts to restore the altered activity of these systems in human pathology, and it could open the possibility of modulating one lysosomal pathway to compensate for failures in the others.